| Abstract | Purpose: To provide results of water calorimetry and ion chamber measurements in high-energy
electron beams carried out at the National Research Council Canada (NRC). There are three main
aspects to this work: (a) investigation of the behavior of ionization chambers in electron beams of different
energies with focus on long-term stability, (b) water calorimetry measurements to determine
absorbed dose to water in high-energy beams for direct calibration of ion chambers, and (c) using
measurements of chamber response relative to reference ion chambers, determination of beam quality
conversion factors, kQ, for several ion chamber types.
Methods: Measurements are made in electron beams with energies between 8 MeV and 22 MeV
from the NRC Elekta Precise clinical linear accelerator. Ion chamber measurements are made as a
function of depth for cylindrical and plane-parallel ion chambers over a period of five years to investigate
the stability of ion chamber response and for indirect calibration. Water calorimetry measurements
are made in 18 MeV and 22 MeV beams. An insulated enclosure with fine temperature control
is used to maintain a constant temperature (drifts less than 0.1 mK/min) of the calorimeter phantom
at 4°C to minimize effects from convection. Two vessels of different designs are used with calibrated
thermistor probes to measure radiation induced temperature rise. The vessels are filled with high-purity
water and saturated with H2 or N2 gas to minimize the effect of radiochemical reactions on the
measured temperature rise. A set of secondary standard ion chambers are calibrated directly against
the calorimeter. Finally, several other ion chambers are calibrated in the NRC 60Co reference field and
then cross-calibrated against the secondary standard chambers in electron beams to realize kQ factors.
Results: The long-term stability of the cylindrical ion chambers in electron beams is better (always
\0:15%) than plane-parallel chambers (0.2% to 0.4%). Calorimetry measurements made at 22 MeV
with two different vessel geometries are consistent within 0.2% after correction for the vessel perturbation.
Measurements of absorbed dose calibration coefficients for the same secondary standard
chamber separated in time by 10 yr are within 0.2%. Drifts in linac output that would affect the transfer
of the standard are mitigated to the 0.1% level by performing daily ion chamber normalization
measurements. Calibration coefficients for secondary standard ion chambers can be achieved with
uncertainties less than 0.4% (k = 1) in high-energy electron beams. The additional uncertainty in
deriving calibration coefficients for well-behaved chambers indirectly against the secondary standard
reference chambers is negligible. The kQ factors measured here differ by up to 1.3% compared to
those in TG-51, an important change for reference dosimetry measurements.
Conclusions: The measurements made here of kQ factors for eight plane-parallel and six cylindrical ion
chambers will impact future updates of reference dosimetry protocols by providing some of the highest
quality measurements of this crucial dosimetric parameter. |
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